Gallium nitride (GaN) based heterostructure field effect transistors (HFETs) have a huge potential for power switches and radio frequency (RF) power amplifiers due to record high operating voltages, peak drain currents, high cut-off frequencies, high operating temperature and robustness. For example, such devices have achieved more than an order of magnitude lower loss in power converters and RF power densities in the range of ten to thirty Watts per millimeter (W/mm), which exceed those achieved with silicon (Si) or gallium arsenide (GaAs) based technologies by a factor of ten to one hundred.
Factors limiting the performance of power GaN HFETs and all other types of field-effect transistors (FETs) include a finite value of the device resistance in the on-state and input/output capacitances in the off-state. A low on-resistance, RON, and low input and output capacitances, CIN and COUT, respectively, are important characteristics of transistor power switches and power amplifiers. In most FET types, such as high electron mobility transistors (HEMTs), (HFETs), metal-semiconductor FETs (MESFETs), metal oxide semiconductor FETs (MOSFETs), etc., the RON value can be decreased by increasing the total gate width, WG. However, a larger total gate width leads to a higher gate capacitance, CG, and gate charge, QG, which increases the switching loss. As a result, a RONCG product provides a figure of merit of a FET-type switch, characterizing the overall power losses. In conventional FETs, the RONCIN and RONCOUT products only can be traded off against the operating voltage, since decreasing RON by shortening the gate-drain distance reduces the maximum device operating voltage.